Advanced coal utilization and conversion processes for power generation, such as the integrated gasification combined cycle (IGCC) employing gas turbines or the gasification-molten carbonate fuel cell (MCFC) power plants, offer higher cycle efficiency, lower CO.sub.2 emissions per kilowatt-hour generated, and the ability to attain more stringent environmental standards for SO.sub.2, NO.sub.x, and particulates than direct coal-fired atmosphere boilers. However, the coal gasifier product gas typically contains several thousand ppm H.sub.2 S. For these advanced coal conversion processes to become commercially feasible, the H2S emission level must be reduced to meet environmental standards, improve process efficiency to competitive levels, and reduce corrosion of the downstream process equipment. Turbines, for example require a level of H.sub.2 S in the fuel gas stream of approximately 10 ppm and fuel cells a level of approximately 1 ppm for proper operation.
Current commercial liquid scrubbing H.sub.2 S removal processes occur at or below ambient temperatures, leading to thermal efficiency losses, and require further treatment of the waste water. Thus, a high temperature gas desulfurization process is needed.
Each of the three types of currently commercially feasible gasifiers, fixed bed, fluidized bed, and entrained bed, operates under different conditions and generates a fuel gas with distinct temperatures, pressures, and gas compositions. Fixed bed gasifiers have outlet temperatures below 650.degree. C., fluidized bed gasifiers as high as 1050.degree. C., while in entrained bed gasifiers, the exit temperature can exceed 1250.degree. C. Water vapor is present in the gas, in amounts depending on gasifier type and the extent of quenching. A representative air-blown fluidized bed-gasifier product gas composition is 54% N.sub.2, 7.5% CO.sub.2, 16% CO, 11% H.sub.2, 0.42% H.sub.2 S, 11% H.sub.2 O, with traces of COS, O.sub.2, and argon. In addition, the mass concentration of ash in this stream is typically 1000 to 5000 ppmw. The presence of ash, hydrogen sulfide, and high concentration of water vapor makes the downstream hot gas clean-up processing particularly challenging.
Throwaway sorbents, such as calcium-based limestone and dolomite have been considered for in-bed (within the gasifier vessel) desulfurization. This is effective in fluidized-bed gasifiers, achieving greater than 90% sulfur removal, and less effective in fixed bed and entrained bed gasifiers, where only 25-60% sulfur removal can occur by CaO-sorbents. Additional problems of fly ash-CaO interaction have been reported, limiting the utility of these throwaway materials. Solid waste disposal and landfill cost increases likely in the near future make these "cheap" sorbents even less attractive.
External desulfurization processes with regenerable, mixed-metal oxide sorbents are presently under development to meet the stringent hot gas cleanup requirements. The attractiveness of these materials arises from their high sulfur removal efficiency, which can exceed 99% for each type of gasifier after adjusting the exit gas temperature to the range appropriate for the chosen sorbent.
Current hot gas desulfurization process options using regenerable sorbents employ fixed bed, fluidized bed, and moving bed systems. However, these systems are subject to a number of problems, which limit their commercial viability, including degradation, e.g., via mechanical attrition, spalling, or decrepitation of both conventional and advanced sorbents, and the resulting problems of downstream process contamination, release of toxic metals to the environment and loss of performance.
For example, fixed bed systems involve changes in operating characteristics during bed switching and variability of flow through the bed. Moving bed systems involve complex solids handling problems, including sealing an oxidizing environment from a reducing environment in the presence of high solids loadings. Fluidized bed systems must operate within a narrow size distribution of sorbent particles to maintain fluidization without loss of sorbent. Also, the particles are subject to severe impaction within the bed, limiting their ability to withstand the many cycles necessary for a commercial process. Sorbent degradation in fixed beds occurs at a lesser rate than in fluidized or moving beds. However, spalling of the sorbent during operation can plug the bed and cause a large pressure drop. Sorbent particle attrition in fluidized and moving beds decreases performance as sorbent fines escape downstream.
Each process also put constraints on the sorbent properties. A trade-off exists between the overall reaction rate and the structural strength of the sorbent particles. This trade-off occurs because the overall reaction is limited both by gas and solid diffusion. For example, in a fixed bed employing pellets, a product layer film forms on the outer surface of the pellet, hindering diffusion of the gas to the unreacted interior. Typically, the metal sulfide product has larger molecular volume than the starting oxide, thereby causing closure of the pores. Low porosity, durable sorbents can be used only at reduced space velocities, requiring larger reactor beds. Making the initial pellet with high porosity would achieve more complete reaction, albeit at the expense of mechanical strength. Production of bulk sorbents with both high macroporosity and good mechanical strength still eludes the art. U.S. Pat. No. 4,977,123 teaches a method for such solids production under specific heat treatment conditions.
Because of the range of gasifier outlet conditions, particularly temperature, many sorbents have been studied for hot gas desulfurization. Early work focused on zinc or iron oxides or combinations of reactants/stabilizers on high surface area substrates. These sorbents react with hydrogen sulfide to form zinc and iron sulfides.
Zinc oxide can achieve a higher degree of gas desulfurization than iron oxide, which, however, is easier to regenerate. The sulfidation reaction of zinc oxide is: EQU ZnO(s)+H.sub.2 S(g).fwdarw.ZnS(s)+H.sub.2 O(g) (1)
Removal to below 15 ppm H.sub.2 S is possible with zinc oxide at temperatures up to 1000.degree. K. for fuel gases containing up to 10% H.sub.2 O vapor. Typically, however, zinc oxide sorbents show low utilization, less than 20% of the theoretical. This is due to formation of a product layer of ZnS around the unreacted oxide core, thus slowing the reaction rate. Particles of high porosity help to alleviate this problem.
A drawback to ZnO has been that, in highly reducing atmospheres, such as in gasifier exit gases with high H.sub.2 and CO contents, zinc oxide is partially reduced to elemental zinc, a liquid above 419.degree. C. which has considerable vapor pressure at the hot gas cleanup temperatures (&gt;550.degree. C.). Thus, zinc can be lost to the system, or condense as a ZnS layer on the outside of the reacting particles. See Lew et al., "Sulfidation of Zinc Titanate and Zinc Oxide Solids," Industrial and Engineering Chemistry Research, Vol. 31, No. 8, p. 1890-1899 (1992).
Zinc vapor and zinc oxides have been disclosed for use at low temperatures (below 600.degree. C.) in conjunction with manganese and its oxides used at high temperatures for desulfurization of a byproduct from injection of coal in a molten iron bath, a process used in steel making. See U.S. Pat. No. 4,852,995. However, such processes do not operate under the same conditions or produce the same gas compositions as with coal gasifiers in the present invention.
Testing of mixed oxide sorbents has also been undertaken, since these sorbents generally offer physical advantages, such as better dispersion and a lower propensity to sintering, and chemical advantages such as activity and regenerability, compared to uncombined active oxides. For instance, zinc ferrite (ZnFe.sub.2 O.sub.4) is a better overall sorbent than either of its constituent oxides and has been extensively studied for hot gas cleanup applications. However, zinc ferrite decomposes into ZnO and Fe.sub.3 O.sub.4 in the reducing conditions of the gasifier outlet. Thus, as with zinc oxide, formation of zinc vapor limits the use of zinc ferrite to a temperature up to 550.degree. C. and to mildly reducing environments.
Zinc titanates have also been studied, since they exhibit slower reduction to volatile zinc than zinc oxide. The sulfidation reaction is: EQU Zn.sub.x Ti.sub.y O.sub.x+2y (s)+xH.sub.2 s.fwdarw.xZnS(s)+yTiO.sub.2 (s)+xH.sub.2 O(g) (2)
However, some reduction of the zinc titanates to Zn vapor also occurs, more so from ZnO-rich compositions. In these zinc titanates, zinc vaporization will gradually enrich the particle surface with a dense zinc sulfide layer from reaction with the zinc vapor and H.sub.2 S, slowing the reaction rate and enhancing undesirable sintering.
Efforts at making sorbents more attrition resistant have also been undertaken. Variations to the several sorbent manufacturing techniques, such as spray drying, impregnation, crushing and screening and granulation, and different binders and chemical additives, such as bentonite and molybdenum, have been studied. Supported sorbents, involving metal oxides and mixed-metal oxides on alumina particles or zeolites, are also under investigation. However, achieving a balance between pore diffusion rate limitations and solid phase reaction rates is currently beyond the state of the art. Therefore, the development of suitable sorbents is presently limiting the commercial development of hot gas desulfurization technology.
Sorbent regeneration also presents difficulties. Regeneration processes may be performed in place with fixed beds. However, sorbent regeneration is an exothermic reaction subject to high bed temperature excursions which can sinter and reduce the effectiveness of the sorbent. Regeneration with fluidized or moving bed systems can be accomplished in separate vessels, allowing for better temperature control, but requires more complex hardware. Also, reducing the temperature below a certain value can lead to the undesirable formation of sulfate species. Additionally, the gas phase concentrations of SO.sub.2 and O.sub.2 at a fixed temperature determine whether sulfates are thermodynamically favored. Sulfate formation is detrimental, as it usually causes severe swelling of the solid, the sulfate having much larger volume than the oxide. This effect will cause spalling or decrepitation of the particle during the next reductive sulfidation cycle. Controlling all three of these variables, temperature, SO.sub.2 and O.sub.2 concentrations, in the fixed bed, fluidized bed, and moving bed processes to avoid both sintering and sulfate formation is difficult. Also, no sorbent has shown regenerability over a commercially useful lifetime.
At the present time, all three types of hot gas desulfurization processes are limited by inadequate sulfur loading on the sorbent and/or poor sorbent stability and regenerability. Further efforts to improve the sorbent performance by chemical modifications and structural strengthening are underway. These are expected to be of marginal impact. Accordingly, a different process design, much less sensitive to the physical properties and/or changes of the selected sorbent, is needed.